The present invention relates to improvements for eliminating the disadvantages of conventional continuous annealing processes of cold rolled strips including water quenching and over-aging treatments, such as the oxidation of the surface of the steel strip during the water quenching and hence the necessity of pickling the steel strip to remove the resulting oxide film on surface.
In a known type of continuous annealing line for cold rolled steel strip involving water quenching and over-aging treatments, a steel strip which has been heated to a temperature between 500.degree. to 800.degree. C through a heating furnance and a soaking pit is quenched in a spray of water, immersed in a pickling tank to remove the oxide film from the surface, and then subjected to an over-aging treatment in a shelf treating furnance.
Several different methods and apparatus have been proposed in which a steel strip is rapidly cooled from an elevated temperature by spraying cooling water against the steel strip. While some of these methods and apparatus take into consideration the flow conditions of water such as laminar flow or turbulent flow, the purpose of this is in all cases to ensure an improved heat transfer coefficient and none of these conventional methods and apparatus take into consideration the flow conditions of cooling water with a view to reducing the amount of surface oxidation of steel strip during the water quenching.
Further, while a method and an apparatus have been proposed by a group of persons including the inventors, in which a steel strip heated to a temperature between 700.degree. to 800.degree. C is quenched without any distortion, in the actual application the surface of the quenched steel strip is oxidized, thus requiring a pickling operation for removing the resulting oxide film from the surface, though there is no occurrence of distortion.